Science
Temperature Conversion Simplified: Celsius, Fahrenheit & Kelvin
A comprehensive guide to temperature conversion — the formulas, the history, and when to use each scale in science, cooking, and everyday life.
Three scales, one quantity. The formulas are short but the mental shortcuts are shorter, and knowing when each scale is appropriate matters more than the arithmetic.
The formulas
- °F = (°C × 9/5) + 32
- °C = (°F − 32) × 5/9
- K = °C + 273.15
- °C = K − 273.15
- °F = (K − 273.15) × 9/5 + 32
Mental-math shortcuts
°C → °F: double it and add 30. Off by a few degrees but close. 20°C → 70°F (real: 68°F). 30°C → 90°F (real: 86°F).
°F → °C: subtract 30 and halve. 80°F → 25°C (real: 26.7°C). 50°F → 10°C (real: 10°C).
Useful anchors: −40 is the same in both scales. 0°C = 32°F. 100°C = 212°F. Body temperature 37°C = 98.6°F.
When to use which
Celsius for everyday weather, cooking outside the US, and most science. Fahrenheit for US weather, US cooking, and HVAC in the US. Kelvin for any physics involving absolute temperature (gas laws, blackbody radiation, thermodynamic cycles).
Critical rule: temperature differences (ΔT) are the same in Celsius and Kelvin but different in Fahrenheit. A 10°C rise = a 10 K rise = an 18°F rise.
Common conversions
- −40°F = −40°C (the crossover)
- 0°F = −17.8°C (very cold day)
- 32°F = 0°C (water freezes)
- 68°F = 20°C (room temperature)
- 98.6°F = 37°C (body)
- 212°F = 100°C (water boils at sea level)
- 350°F = 177°C (typical baking)
- 0 K = −273.15°C (absolute zero)